Fused salt baths-composition and method for carburising



United States Patent 3,117,038 FUSED SALT BATHSCOMPOSITION AND METHOD FOR CARBURISING Robert Leslie Hewson and Frederick David Waterfall, Oldbury, England, assignors to imperial Chemical Industries Limited, London, England, a corporation of Great Britain No Drawing. Filed Nov. 9, 1961, Ser. No. 151,171 Claims priority, application Great Britain Nov. 14, 1960 12 Claims. (Cl. 14S15.5)

The present invention relates to improvements in or relating to fused salt baths. More particularly it relates to fused salt baths for the carburisation of iron or steel and to a process for the carburisation of iron or steel therein.

It is known to introduce carbon into the surface layers of iron or steel parts by immersing the parts in a molten salt mixture containing essentially a cyanide of an alkali metal, usually sodium cyanide. For small depths of penetration of carbon into the parts it is usual to employ a salt bath containing up to about 50% of alkali metal cyanide, the remainder consisting of alkali metal carbonates or a mixture of alkali metal carbonates and halides. When greater depths of carbon penetration are required, and par ticularly when maximum saturation of the surface layers of the metal with carbon is needed, as, for example, to allow for grinding the surface of the treated parts, it is known to employ accelerated carburising baths which contain alkali metal cyanides and halides together with about 4055% alkaline earth metal halide, usually barium chloride, as accelerator. Such accelerated baths contain about 10-20% alkali metal cyanide and are usually operated in the temperature range 850950 C. Since it is known that the rate of carburising of iron and steel increases as the temperature of the carburising bath is raised, the highest permissible temperature is used when the most rapid carburising is desired. With the accelerated carburising baths aforementioned this temperature lies in the range 930950 C. since with these baths frothing and fuming become too severe and loss of salt becomes excessive if a temperature of 950 C. is exceeded.

We have now found that carburisation of iron and steel can be carried out at temperatures higher than 950 C. in a salt bath comprising alkaline earth metal halide, alkaline earth metal carbonate, alkali metal cyanide and alkali metal carbonate, and that the speed of carburisation is considerably higher under these conditions than in the aforementioned accelerated carburising baths of the prior art operated at their maximum working temperature of 930950 C.

According to the present invention, a salt bath suitable for the carburisation of iron and steel at temperatures in the range 9501l00 C. comprises by weight alkaline earth metal halide 65-72%, alkaline earth metal carbonate l5%, alkali metal cyanide 210%, and a balance which is alkali metal carbonate.

The alkaline earth metal salts employed are most suitably the barium salts. We prefer to employ a bath containing about 70% alkaline earth metal halide and about alkaline earth metal carbonate. For example a bath according to the invention containing 70% barium chloride and 10% barium carbonate gives very satisfactory results when used for carburising in the temperature range 950-1100 C. We prefer to operate the bath at a 3,117,038 Patented Jan. 7, 1964 temperature of 1000-1100 C. since these high temperatures give a very rapid rate of carburising and enable the greatest case depths to be obtained in the shortest possible times. For most classes of work we prefer to employ about 35% by Weight of alkali metal cyanide in the bath, for example 35% of sodium cyanide.

The salt baths of the present invention are made up by melting together the salt components and raising the molten bath to working temperature. Before the bath is raised to working temperature, the surface may be covered with a layer of graphite, as is known in salt bath practice, in order to prevent undue loss of cyanide salt by atmospheric oxidation. With these high temperature baths, however, the oxidation of cyanide is better controlled by providing the salt bath with a sealed hood such as a bellshaped cover and passing a non-oxidising gas into and through the hood. The non-oxidising gas may for example be coal gas, carbon monoxide, methane or nitrogen. When the carburising time is not more than about 2 hours the non-oxidising'atmosphere may be maintained above the bath throughout the time that the parts are in the bath, as sufiicient oxygen to maintain the carburising reaction enters each time the parts are removed and replaced by a new charge. When the carburising time is longer, however, it is an advantage to allow a small amount of oxygen to'enter the hood or even to remove the hood from the bath momentarily during the carburising period. This procedure ensures the entry of suflicient oxygen into the bath to maintain the carburising reaction but avoids wastage 'of cyanide by excessive oxidation. It is advantageous to maintain a layer of graphite on the bath surface in addition to the non-oxidising atmosphere above the bath. The graphite layer serves to reduce oxidation and fuming of the bath when parts are being introduced into or removed from it.

During operation of the salt baths the cyanide content gradually falls and some salt is also removed on the surface of the treated iron or steel parts. It is therefore necessary to add a make-up salt at intervals to maintain the cyanide strength and to restore the bath level. his not necessary to addalkali metal carbonatein the makeup salt as this tends to be formed continuously by oxidation of cyanide. We have found that a make-up salt comprising by weight alkaline earth metal halide -72%, alkaline earth metal carbonate 5-1-5 alkali metal cyanide 1-525% is suitable for maintenance of the salt baths of the invention.

The following examples further illustrate but do not limit the invention.

Example 1 A salt mixture of composition BaCl 10% BaCO 5% NaCN, 15% N21 CO by weight was melted, covered with a layer of graphite and raised to a working temperature of 1050 C. Pieces of carbon case-hardening steel 'Were carburised by immersing them in the bath for either 1 hour or 2 hours at 1050 C. The pot containing the salt bath was covered by a sealed bell through which coal gas was passed at all times except during introduc tion and removal of the steel pieces, the gas being burned at the exit from the bell. The bath was in use for 60 days, the cyanide content being maintained in the range 3-5 NaCN by addition each day of a make-up salt mixture of composition 70% BaCl, 10% BaCO 20% NaCN by weight. The treated steel pieces were cooled in air and examined for case depth. It will be seen from the following table of results that the salt bath according to the invention gave surprisingly rap-id carburisation with Example 2 A fused salt bath of composition by weight 70% BaCl, B aCO 3.4-4.5% NaCN, balance Na CO was employed at a temperature of approximately 1100 C. for carburising parts in carbon caseharden-in-g steel to B5. Specification En. 32 by soaking the parts therein for times of /2 hour to 4 hours. An atmosphere of coal gas was maintained over the bath surface as in Example 1. The parts were slowly cooled after treatment and examined metallographically. Results obtained during the 70th to the 73rd day of operation of the bath are shown in the following table.

Carburis- Case Depth, in. Day of Test ing Time,

Eutectoid Total 0. 020 0. 055 1 0. 029 0. 005 3 0. 022 0. 046 1 0. 033 0. 063 2 0. 043 0. 080 A 0. 020 O. 051 1 0. 033 0. 065 2 0. 048 0. 087 4 0. 060 0. 118 as 0. 022 0. 046 1 0. 035 0. 066 2 0. 046 0. 089 4 0. 058 0. 119

Whatwe claim is:

1. A process for the carburization of ferrous metal with a fused salt bath at high carburization rates while maintaining fuming and cyanide loss at a minimum which comprises heating the metal in a fused salt'bath consisting essentially of, by weight, alkaline earth metal halide 70%, alkaline earth metal carbonate 10%, alkali metal cyanide 3-5 balance alkali metal carbonate at a bath temperature of 950 C.-10C C.

2. A process as in claim 1 which further comprises providing a non-oxidizing atmosphere over the surface of said bath during at least part of said carburization process.

3. A salt composition according to claim 12 which consists essentially of by weight substantially 70% barium chloride, 10% barium carbonate and 20% sodium cyanide.

4. A process for the carburisation of ferrous metal which comprises heating the metal in a fused salt bath which consists essentially of by weight alkaline earth metal halide -77%, alkaline earth metal carbonate S-15%, alkali metal cyanide 2-10%, and a balance which is alkali metal carbonate at bath temperature of 95 0-1100 C.

5. A process according to claim 4 wherein the bath temperature is 1000-1100 C.

6. A process as in claim 4 which further comprises providing a non-oxidizing atmosphere over the surface of said bath during at least part of said carburization process.

7. A fused salt bath for the carburisation of iron or steel, which consists essentially of by weight alkaline earth metal halide 65-72%, alkaline earth metal carbonate 5-15%, alkali metal cyanide 210%, and a balance which is alkali metal carbonate.

8. A fused salt bath according to claim 7, wherein the alkaline earth metal halide is barium chloride and the alkaline earth metal carbonate is'bariurn carbonate.

9. A fused salt bath according to claim 7, wherein the alkaline earth metal halide is substantially and the alkaline earth metalcarbonate is substantially 10% by weight of the bath.

10. A fused salt bath according to claim 7, wherein the alkali metal cyanide is substantially 3-5 by weight of the bath.

11. A fused salt bath according to claim 10, wherein the alkali metal cyanide is sodium cyanide.

12. A salt composition suitable for the maintenance of a fused salt bath, which consists essentially of by weight alkaline earth metal halide 65-72%, alkaline earth metal carbonate 5-15% and alkali metal cyanide 15-25%.

References Cited in the file of this patent UNITED STATES PATENTS 1,980,152 Beck Nov. 6, 1934 1,993,204 Beck Mar. 4, 1935 2,095,188 Hanusch Oct. 5, 1937 2,169,008 Sarvis Aug. 8, 1939 2,342,104 Holt Feb. 22, 1944 

1. A PROCESS FOR THE CARBURIZATION OF FERROUS METAL WITH A FUSED SALT BATH AT HIGH CARBURIZATION RATES WHILE MAINTAINING FUMING AND CYANIDE LOSS AT A MINIMUM WHICH COMPRISES HEATING THE METAL IN A FUSED SALT BATH CONSISTING ESSENTIALLY OF, BY WEIGHT, ALKALINE EARTH METAL HALIDE 70%, ALKALINE EARTH METAL CARBONATE 10%, ALKALI METAL CYANIDE 3-5%, BALANCE ALKALI METAL CARBONATE AT A BATH TEMPERATURE OF 950*C.-100*C. 